GEMS is an error classification scheme developed by Reason that focuses on cognitive factors in human error as opposed to environmental or other context-related factors. It is based heavily on Rasmussen’s three major categories of errors: skill-based slips and lapses, rule-based mistakes, and knowledge-based mistakes (SRK). GEMS is a more general description of the cognitive “black box”, which can be used to address the mechanisms of both slips and mistakes. GEMS taxonomy of error types is a useful method to assess cognitive determinants in complex technological environments.